Wanting the results of the well-known especially-sensitive leakage tests of the Sencore Mighty-Mites, I started by looking in the setup book for my TC-109, my always-out go-to tester.

No listing for the 7F8 ( loctal for the uninitiated ).

So, I dragged out my recently-restored TC-162 and looked up the 7F8. Sure enough, the tube setup was listed as were many other 7-series loctals.

Here's where the plot thickens. The socket specified for the 7F8 is socket 1, while all the others show socket 9, the only loctal socket on the TC-162.

Socket 1 is an octal socket, apparently used when, like the 7F8, the heater connection is non-standard.

After checking the tube manual again, I decided to see if I could test the tube in the octal socket. By rotating the tube slightly once it was inserted, I was able to get the pins to make contact with only gentle continuous pressure in the rotated direction.

I was able to test the tube that way, but I'm sure that's not the way things were meant to be.

Sitting here tonight, a couple weeks later, I got to thinking perhaps there was an adapter furnished to accomplish this test, although one would think such would be mentioned in the setup info if that were the case. I don't think it could be a misprint because of the way the sockets are wired.

The reason I thought of a Sencore-supplied adapter is because my mind was trying to think of how I might make an adapter for such, as I already have at least two boatanchors that use this wonderful prima Donna tube.

In every tube tester I've worked with, the pins of all sockets are connected in parallel corresponding to pin number. So, If your tube tested successfully in an octal socket, then I think it probably would test successfully in the loctal socket. Maybe it was just a typo in the data book, and they specified the wrong socket.

Unfortunately, Bob, that's not the case. It's not a typo. Socket 9 will not work because of the filament wiring scheme.

Socket 1 has the filaments wired as needed by the 7F8. Socket 9 has the filaments wired as needed by most other Loctals as well as most other octals.

I initially followed your line of thinking when I first attempted testing the 7F8 and it of course showed a short right away on pin 2. I don't remember if others showed shorted, but I know the filaments also did not light up.

In every tube tester I've worked with, the pins of all sockets are connected in parallel corresponding to pin number. So, If your tube tested successfully in an octal socket, then I think it probably would test successfully in the loctal socket. Maybe it was just a typo in the data book, and they specified the wrong socket.

The Mercury 2000 is the same as what he's dealing with. On that one, I have 4 octal sockets, a pair of 7 pin sockets and 4 9 pin. Tubes with the heaters on different pins go to different sockets instead of having some way to select what pis that are the heater line. Seems like having a switch like most other testers would have been better, but I'm sure they had some reason why not.